1. Field of the Invention
This invention relates to an automatic focusing device and more particularly to an automatic focusing device adapted for a video camera.
2. Description of the Related Art
The known automatic focusing devices for image pickup apparatuses such as video cameras or the like include an automatic focusing device of a so-called hill-climbing type. The hill-climbing type automatic focusing device is arranged to carry out focusing in the following manner. A signal representing a high frequency component is extracted out of a video signal obtained from an image sensor which is a CCD or the like. A focusing lens is driven to move in such a way as to bring the level of this signal of high frequency component to a maximum level. The automatic focusing device of this type obviates the necessity of any additional optical member otherwise to be used in focusing and is capable of accurately focusing on an object of shooting located at any distance irrespective as to whether the distance is far or near.
FIG. 1 shows in a block diagram the arrangement of the automatic focusing device of the above-stated kind. Referring to FIG. 1, a focusing lens 1 is arranged to be movable in the direction of an optical axis. An optical image of an object of shooting coming through the focusing lens is formed on a pickup image plane of an image sensor 2. The image sensor 2 which is a CCD or the like is arranged in the rear of the focusing lens 1 to photo-electrically convert the optical image into a video signal. A sample-and-hold circuit (hereinafter referred to as a CDS circuit) 3 is connected to the image sensor 2 and is arranged to sample and hold the video signal coming from the image sensor 2. An automatic gain control (hereinafter referred to as AGC) circuit 4 is connected to the CDS circuit 3 and is arranged to control and bring to a predetermined level the gain of the video signal which is sampled and held by the CDS circuit 3. An analog-to-digital (hereinafter referred to as A/D) converter 5 is connected to the AGC circuit 4 and is arranged to convert the video signal from an analog form into a digital video signal. To the A/D converter 5 is connected a camera signal processing circuit 6 which is arranged to convert the digital video signal into a standard TV signal. A band-pass filter (hereinafter referred to as BPF) 10 which is arranged to extract a high frequency component of the digital video signal is also connected to the A/D converter 5. The high frequency component varies with the state of focus within the video signal.
To the BPF 10 is connected a gate circuit 11. The gate circuit 11 is arranged to pass a portion of the high frequency video signal which comes from the BPF 10 and corresponds to a focus detecting area of the image plane, in accordance with a gate control signal obtained from a gate control signal forming circuit 16. To the gate circuit 11 is connected a detection circuit 12 which is arranged to hold the peak of the output of the gate circuit 11 at intervals synchronized with a period of time which is an integer times as long as a vertical synchronizing signal. To the detection circuit 12 is connected a motor control circuit 13 which is arranged to set the direction and the speed of a driving action of a motor 15. To the motor control circuit 13 is connected a motor driver 14, which is connected to the motor (M) 15. The motor 15 is arranged to drive the focusing lens 1.
The automatic focusing device arranged in this manner operates as follows. The focusing lens 1 forms an optical image of an object of shooting on the pickup image plane of the image sensor 2. The optical image is photo-electrically converted by the image sensor 2 into a video signal. The video signal which represents the object image is supplied to the CDS (sample-and-hold) circuit 3 to be sampled and held. The sampled-and-held video signal is supplied to the AGC circuit 4 to be amplified up to a predetermined level. The amplified video signal is converted by the A/D converter 5 into a digital video signal. The digital video signal is supplied to the camera signal processing circuit 6 to be converted into a standard TV signal. The standard TV signal is supplied to external circuits such as a video tape recorder (VTR) and a monitor or the like. The digital video signal is supplied also to the BPF 10. The BPF 10 extracts the high frequency component out of the video signal. The high frequency component extracted is supplied to the gate circuit 11. The gate circuit 11 operates, on the basis of the gate control signal obtained from the gate control signal forming circuit 16, to take out from the output of the BPF 10 only the high frequency component of a video signal portion which corresponds to the focus detecting area which is set within the image plane. The high frequency component of the video signal taken out from the gate circuit 11 is supplied to the detection circuit 12 to be peak-held at the intervals synchronized to a period of time which is an integer times as long as the period of the vertical synchronizing signal. As a result of the peak holding action, an automatic focusing (hereinafter referred to as AF) evaluating value signal is obtained.
The AF evaluating value signal is supplied to the motor control circuit 13. The motor control circuit 13 then sets the motor driving direction in which the AF evaluating signal increases. The motor control circuit 13 also sets the motor driving speed, i.e., a focusing speed, at a high speed in the event of a greatly blurred state of the object image and at a low speed in case of a slightly blurred state. The motor driver 14 then drives and controls the motor 15 in the direction and at the speed set by the motor control circuit 13. In accordance with the direction and the speed as set, the motor 15 causes the focusing lens 1 to move along its optical axis. The so-called hill-climbing type automatic focusing action is carried out in this manner.
In the conventional automatic focusing device described above, the gate circuit 11 is arranged to operate under the control of the gate control signal which is formed by the gate control signal forming circuit 16 to control and cause the gate circuit 11 to open and close in such a way as to pass only the signal that corresponds to the focus detecting area which is fixedly set beforehand within the image plane. As a result, only a portion of the high frequency component of the video signal coming from the BPF (band-pass filter) 10 that corresponds to the fixed focus detecting area is used for focus adjustment. Therefore, when the whole image plane is shaken by shaking of images, etc., the focus detecting area also shakes to cause an image obtained within the focus detecting area either to suddenly move or to move out of and into the focus detecting area or to cause some undesired image to come into the focus detecting area. In such a case, the value of the AF evaluating value signal abruptly changes to make the automatic focusing action unstable.
Image-shake correcting methods which have been developed for solving this problem of shaking of images include two methods. One is a method of correcting the shaking of images by moving the optical system on the basis of vibrations detected. The other is a method of correcting the picked-up image signal on the basis of the shake of images detected. The optical-system moving method has a merit in that the signal used for automatic focusing (AF) is also corrected but has a demerit in that the number of necessary parts increases to make the arrangement of the device more complex. The method of correcting the picked-up image signal on the basis of the shake of images detected, on the other hand, is not effective for correcting the unstableness of the automatic focusing action, because the correction by this method is generally made after the signal to be used for automatic focusing is taken out.
In view of the present state of the automatic focusing action described in the foregoing, it is an object of this invention to provide an automatic focusing device which prevents the automatic focusing action from becoming unstable due to shaking of images and is capable of stably focusing on a main object of shooting.
To attain this object, an automatic focusing device which is arranged according to this invention as a preferred embodiment thereof includes motion vector detecting means for detecting from a picked-up image signal outputted from image pickup means a movement of a picked-up image taking place within an image plane, filter means for extracting a signal of a predetermined frequency component from the picked-up image signal, gate means for taking out from the signal extracted by the filter means a signal corresponding to a focus detecting area in the image plane, area setting means for correcting the position of the focus detecting area by controlling the gate means on the basis of a detection output of the motion vector detecting means, and driving means for driving a focusing lens to move to an in-focus position on the basis of an output of the gate means.
To attain the same object, an automatic focusing device which is also arranged according to this invention as another preferred embodiment thereof includes motion vector detecting means for detecting from a picked-up image signal outputted from image pickup means a movement of a picked-up image taking place within an image plane, image-plane cutting-out means for varying a range of cutting out an image plane from the picked-up image signal on the basis of a detection output of the motion vector detecting means in such a way as to stabilize an image of an object of shooting, filter means for extracting from an output of the image-plane cutting-out means a signal of a predetermined frequency component, gate means for taking out from the signal extracted by the filter means a signal corresponding to a focus detecting area in the image plane, and driving means for driving a focusing lens to move to an in-focus position on the basis of an output of the gate means.
The embodiment which is arranged in the above-stated manner operates as follows. A movement of a picked-up image that takes place within an image plane is detected by the motion vector detecting means on the basis of, for example, a computing operation on a time-series correlation between images. The filter means extracts the signal of the predetermined frequency component from the picked-up image signal. The extracted frequency component signal is inputted to the gate means. The gate means then takes out the signal corresponding to the focus detecting area in the image plane from the signal extracted by the filter means. Then, the position of the focus detecting area is corrected on the basis of the detection output of the motion vector detecting means. The driving means drives the focusing lens to an in-focus position on the basis of the output of the gate means.
In the case of another embodiment, the motion vector detecting means detects a movement of the picked-up image taking place within the image plane on the basis of a computing operation on a time-series correlation between images. The image-plane cutting-out means acts, on the basis of the detection output of the motion vector detecting means, to vary the cutting-out position of the image plane to be cut out from the picked-up image signal. The image-plane cutting-out position is varied to correct the movement in such a way as to stabilize the object image. Further, the filter means extracts, from the output of the image-plane cutting-out means, the signal of the predetermined frequency component. The gate means then takes out, from the signal extracted by the filter means, a signal corresponding to the focus detecting area in the image plane. The driving means drives the focusing lens to move to an in-focus position on the basis of the output of the gate means.
The above and other objects and features of this invention will become apparent from the following detailed description of embodiments thereof taken in connection with the accompanying drawings.